Engines operating with a variable number of active or deactivated cylinders may be used to increase fuel economy, while optionally maintaining the overall exhaust mixture air-fuel ratio about stoichiometry via cylinder valve deactivation. In some examples, half of an engine's cylinders may be deactivated during selected conditions, where the selected conditions can be defined by parameters such as speed/load window, as well as various other operating conditions including vehicle speed.
However, in some engine/vehicle combinations, only modest fuel economy gains may be possible. Various factors may limit the potential fuel economy gain, such as transition constraints, noise vibration harshness constraints, and others. Further, these factors can serve to reduce the available window of VDE operation, thus further reducing potential gains. While various approaches of the skilled engine designer are aimed at improving these limitations via improved design and system optimization, fuel economy gains may nevertheless be difficult to realize in practice.
The inventor herein has recognized a significant potential constraint on the available operating window may be related to knock. For example, when trying to expand VDE operation in some systems, the engine may become knock limited during various conditions, such as higher load operation. As such, when VDE operation is utilized, the engine cylinders that continue combustion may be forced into a speed/load region (e.g., higher load) different than otherwise experienced at full cylinder conditions and thus may increase a tendency toward knock. As a result, ignition timing retard, used to mitigate knock, can generate a fuel economy loss that possibly outweighs the gains of VDE operation.
Again, while various approaches may be used to optimize engine design in an attempt to mitigate engine knock, for example, such optimizations typically involve still other trade-offs.
The above issues and trade-offs may be addressed by a flex-fuel (e.g., alcohol, ethanol, E-85, gasoline) variable cylinder combustion engine. For example, the system may adjust VDE operation, such as the VDE operating window, to take advantage of the knock abatement properties of an alcohol-blended fuel when present in the vehicle. In this way, not only can the vehicle operate on a variety of fuel blends, but the partial cylinder deactivation of the engine can be coordinated with the presence and blend of the fuel, thereby providing improved performance when possible. Further, the engine may include direct injection or port injection with at least some open valve injection to further exploit charge cooling characteristics of the blend and further abate knock thus enabling expanded VDE operation.
In another example, the engine control system may adjust the fuel blend during operating conditions to provide knock abatement when needed to exploit the synergistic effect of the fuel blend and partial cylinder deactivation. For example, a directly injected fuel blend with a variable blend of gasoline and alcohol may be used to enable extended VDE operating conditions (e.g., at higher engine loads) by advantageously utilizing variable charge cooling matched to the current engine conditions.
In this way, it may be possible to realize greater fuel economy gains from partial cylinder operation, while also enabling further benefits of alternative fuels.